The present invention relates to a method for regenerating exhausted zeolitic catalysts containing Ti, whose catalytic performances cannot be re-established by the treatment described in the known art.
In particular, the method relates to the regeneration of exhausted catalysts consisting of microporous crystalline materials with an MFI-type structure containing titanium and originally having a composition corresponding to formula (I):
xTiO2xc2x7(1xe2x88x92x)SiO2xe2x80x83xe2x80x83(I)
wherein x ranges from 0.0001 and 0.04 and preferably from 0.01 to 0.03.
Catalysts having general formula (I) can be prepared according to the methods described in U.S. Pat. No. 4,410,501 and, in the form of microspheres with a high crushing strength, in U.S. Pat. Nos. 4,954,653 and 4,701,428.
The properties and main applications of said compounds (titanium silicalites, TS-1) are well known in the art (B. Notari; Structure-Activity and Selectivity in Heterogeneous Catalysis; R. K. Grasselli and A-W. Sleight Editors; Elsevier; 1991; pages 243-256).
These catalysts are typically used in direct oxidation reactions with hydrogen peroxide of organic substrates, such as aromatic hydrocarbons (U.S. Pat. No. 4,369,783), olefins (EP 100,119), alcohols (U.S. Pat. No. 4,480,135), nitrogenated compounds (U.S. Pat. No. 4,918,194; U.S. Pat. No. 5,320,819) and in ammoximation reactions of carbonyl compounds (U.S. Pat. No. 4,794,198; EP 496,385).
The deterioration in the catalytic performances of titanium silicalites, after a more or less prolonged use in the above reactions, can be caused by many factors; for example, it may be due to impurities present in the reagents or migrating from the equipment or to reaction by-products which accumulate in the micropores of the material.
The most well known method in the art for regenerating exhausted zeolitic catalysts consists in subjecting them to thermal treatment under controlled gaseous stream (air, nitrogen/air) and temperature conditions; for example, said treatment can be carried out at temperatures ranging from 500 to 800xc2x0 C. first in a stream of nitrogen and then in air.
In another process (WO 98/18556), preferably applied to the TS-1 used in epoxidation reactions of olefins and allyl chloride, the thermal treatment is carried out at temperatures lower than 400xc2x0 C.
The regeneration of catalysts by thermal treatment alone, however, is not very effective if the deactivation of the zeolitic material is caused, for example, by impurities which are difficult to eliminate by combustion with air or caused by more or less marked alterations in the composition and/or structure.
In these cases, in order to avoid the substitution of the catalyst and consequent rise in costs, resort is made to chemical regeneration methods.
Italian patent application 22413 A/89 describes a method for regenerating titanium silicalites, which consists in effecting a treatment with a nitrogenated organic base under hydrothermal conditions, preferably in the presence of an acid selected from citric acid, tartaric acid and formic acid, subsequent to the thermal treatment.
In the patent (WO 98/18555), the treatment of titanium silicalite is carried out with aqueous or organic solutions of oxidizing agents preferably selected from hydrogen peroxide, ozone and organic peroxide compounds; this method is particularly suitable for the regeneration of TS-1 used in epoxidation processes of olefins, the hydroxylation of aromatic compounds and the oxidation of saturated hydrocarbons.
A process has now been found which allows the regeneration of exhausted catalysts, originally having general formula (I), whose catalytic performances cannot be re-established by thermal treatment alone.
In particular, the object of the present invention relates to a process for the regeneration of exhausted catalysts consisting of microporous crystalline materials with an MFI-type structure containing titanium and having a composition corresponding to formula (I):
xTiO2xc2x7(1xe2x88x92x)SiO2xe2x80x83xe2x80x83(I)
wherein x ranges from 0.0001 to 0.04 and preferably from 0.01 to 0.03, which consists in subjecting the catalyst, previously calcined, to a treatment in an aqueous medium with hydrogen peroxide, in the presence of fluorinated inorganic compounds.
The process is generally carried out within a pH range of 3.5 to 4.5 and at temperatures ranging from 50 to 100xc2x0 C.
The process of the present invention, although being generally valid for the regeneration of exhausted catalysts coming from oxidation processes of organic substrates with hydrogen peroxide, typical of titanium silicalite (TS-1), has proved to be particularly useful for regenerating catalysts deriving from the oxidation of nitrogenated basic compounds such as, for example, secondary amines (U.S. Pat. No. 4,918,194) and ammonia (U.S. Pat. No. 5,320,819) or ammoximation reactions of carbonyl compounds, such as for example, cyclohexanon (U.S. Pat. No. 4,794,198; EP 496,385).
The deterioration in the activity of TS-1 in the above reactions is known in literature (G. Petrini et al., Deactivation Phenomena on Ti-silicalite; Catalyst Deactivation 1991; C. H. Bartholomew, J. B. Butt (Editors); Elsevier 1991, pages 761-766).
It is caused by the combination of three main phenomena consisting: a) in the gradual dissolution of the structure with the accumulation of Ti on the surface of the solid; b) in the removal of the Ti from the structure and c) in the filling of the pores with reaction by-products. As a result of these phenomena, the Ti-silicalite undergoes a progressive alteration in the composition and structure which, as the reaction proceeds, leads to a gradual loss in activity until a level is reached which is unacceptable for the economy of the process. This normally takes place when the catalyst, from a chemical point of view, has a Ti content which is significantly higher than the initial content (for example 1.5-2 times higher).
In spite of this considerable alteration in composition, when the regeneration method, object of the present invention, is applied to the above exhausted catalysts, it surprisingly allows at least 80% of the initial catalytic activity to be recuperated, which greatly benefits the economy of synthesis processes.
The regeneration method, object of the present invention, preferably relates to exhausted catalysts based on Ti-silicalites coming from oxidation reactions of nitrogenated basic compounds or the ammoximation of carbonyl compounds, said materials being characterized by a molar ratio Ti/Si higher than that of fresh catalysts having general formula (I). The regeneration method, carried out on the exhausted material previously subjected to thermal treatment according to the known art, basically consists in a dissolution reaction of Ti; this is effected at temperatures preferably ranging from 60 to 90xc2x0 C. in an aqueous medium with hydrogen peroxide in the presence of fluorinated inorganic compounds.
This reaction can be carried out by initially preparing a suspension of the calcined catalyst, for example from 5 to 15% by weight, in the aqueous solution of the reagents (hydrogen peroxide, fluorinated compound) and then thermostat-regulating at the selected temperature. According to another embodiment of the method, the hydrogen peroxide can be added to the suspension of the catalyst in the solution of fluorinated compound, already thermostat-regulated; furthermore, the dosing of hydrogen peroxide can be effected by a single addition or several additions over a period of time.
The fluorinated compounds useful for the purposes of the present invention can be selected from hydrofluoric acid, ammonium fluorides or fluorides of alkaline metals; in addition inorganic fluoroderivatives soluble in water can be used, in acid form such as, for example, fluosilicic and fluorboric acids, or in salified form with NH4OH.
Among the above fluorinated compounds, ammonium derivatives are preferred; ammonium fluoride (NH4F) and difluoride (NH4HF2) are particularly preferred.
Hydrogen peroxide in aqueous solution at 30% by weight is typically combined with the fluorinated compounds.
The concentration of reagents in the reaction medium is defined in relation to the concentration and chemical composition of the exhausted catalyst. As specified above, controlling the dissolution reaction of Ti is the critical point of the regeneration method, object of the present invention; in the case of exhausted materials characterized by Ti/Si molar ratios higher than those of fresh catalysts, the concentration of the reagents and the operating conditions (temperature and treatment time) are defined so as to approximately re-establish the original chemical composition.
It has been observed, in fact, that a continuous recovery of the catalytic activity of the materials corresponds to a progressive dissolution of Ti in the reaction medium. This effect is illustrated in FIG. 1 which relates to the series of regeneration tests of an exhausted catalyst deriving from the synthesis of cyclohexanone-oxime (CEOX) from cyclohexanone; this Figure indicates the turn-over values (TOH=CEOX moles/Ti moles-hour) in relation to the TiO2 content in the regenerated catalysts.
The recovery of at least 80% of the catalytic activity takes place in correspondence with the chemical compositions more or less the same or slightly lower than that of the fresh catalyst.
In accordance with this trend, the treatment conditions should therefore be established each time in order to control the dissolution degree of Ti within a suitable range of values. For example, in the case of the regeneration of exhausted catalysts coming from oxidation processes of nitrogenated compounds or the ammoximation of carbonyl compounds and normally characterized by a Ti content approximately double of that of fresh catalyst, the treatment conditions are selected so as to dissolve about 50% of Ti.
The quantity of reagents (fluoride, hydrogen peroxide) is usually dosed in relation to the Ti content (in moles) in the exhausted catalyst. Expressing the concentration of the fluorinated compound in moles of F, the molar ratio F/Ti can vary from 1.0 to 3.5; under the preferred conditions in which ammonium fluoride or difluoride are used, this ratio normally ranges from 1.5 to 3.0.
With respect to the dosage of hydrogen peroxide, it should be noted that this is also influenced by the catalytic activity of the exhausted material in the decomposition reaction of H2O2. This reagent, in fact, as well as participating in the dissolution reaction of Ti in the form of peroxide compound, is subject to degradation due to the zeolitic Ti. Under the operating conditions of the process, object of the present invention, the quantity of hydrogen peroxide used is such that the molar ratio H2O2/(soluble Ti) in the mother liquor at the end of the reaction, is at least equal to 1.0 and is preferably higher than 2.0. This implies an initial molar dosage of H2O2/Ti varying from 5.0 to 30; under the preferred conditions in which ammonium fluoride or difluoride are used, this ratio normally ranges from 10 to 25.
The regeneration reaction is conveniently carried out at a temperature ranging from 50 to 100xc2x0 C., preferably from 60 to 90xc2x0 C. and with residence times, under constant temperature conditions, ranging from 1 to 6 hours.
At the end of the treatment, the catalyst is separated from the reaction medium and washed repeatedly with deionized water.
The material is then dried, for example, at a temperature of 110-120xc2x0 C.; the drying can also be followed by a thermal treatment in air at a temperature of 400 to 600xc2x0 C. to ensure the complete removal of possibly absorbed ions, such as the ammonium ion, for example.
Some examples are provided hereunder relating to the regeneration, according to the method of the present invention, of an exhausted catalyst based on Ti-silicalite and coming from the synthesis of cyclohexanone-oxime (CEOX).
The catalysts treated according to the examples were then subjected to a catalytic activity test in the ammoximation reaction of cyclohexanone with ammonia in the presence of hydrogen peroxide.